Electronics > Projects, Designs, and Technical Stuff
Howland Current Pump
JoelK:
Hello everyone,
my master thesis has the advanced howland current pump as topic. I work since quite a long time on the topic because it started as a small project.
I don't just use a single advanced howland current pump, but two identical pumps in a bridge configuration so that one drives and one sinks the AC current at a time. There is also a common mode feedback, which does not really matter in this case. But what matters is an addtional opamp which I use to buffer the positive feedback path. This basically allows me trimming which I need due to the bridge configuration. The problem this addtional opamp brings, is its input impedance which is directly connected to the output of each current pump. This limits the output impedance of my current pump. Most FET input opamps do good at low frequencies but what I look for is a really low input capacity opamp. At the moment I use the LTC6268 which has 0.45pF common mode and 0.1pF differential input capacity. High GBW and high open loop gain is also important but at the moment the input impedance of the LTC6268 dominates all my results. Does anyone know an opamp with less input capacity? Analog Devices shows all the required data one their website, TI is not so nice.
Best regards and nice monday :/
Joel
Zero999:
Schematic?
Are you talking about something similar to the circuit linked below, but without the booster transistors?
https://www.eevblog.com/forum/projects/need-help-with-bi-directional-constant-current-source-(100ma)/msg1972685/#msg1972685
JoelK:
--- Quote from: Zero999 on December 09, 2019, 10:29:21 pm ---Schematic?
Are you talking about something similar to the circuit linked below, but without the booster transistors?
https://www.eevblog.com/forum/projects/need-help-with-bi-directional-constant-current-source-(100ma)/msg1972685/#msg1972685
--- End quote ---
Yes exactly, without the current booster.
https://www.researchgate.net/profile/Pedro_Bertemes-Filho/publication/258327515/figure/fig5/AS:297527111897089@1447947399054/Schematic-diagram-of-the-mirrored-modified-Howland-current-source.png This schematic with an additional opamp in each positive feedback path.
JoelK:
Sorry, for clarity
jbb:
How much DC precision and bandwidth do you need? A bootstrapped JFET buffer might be an option...
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